GB2126829A - Image correction - Google Patents

Abstract

Conventionally image defects arising in a camera tube have been corrected by the application of a standard correction signal predetermined from mainly theoretical considerations. This invention measures the values of a parameter from each of the picture elements, calculates a correction signal from the parameter values and applies the correction signal to correct the image defects. Calculation of the correction signal preferably includes multiplying each of two adjacent line or field parameter values (VALUE 1, VALUE 2) by a respective reference signal (V REF. 1, V REF. 2) which has a frequency equal to half the frequency of occurrence of the picture elements whose values are being multiplied, the reference signals (V REF. 1, V REF. 2) being in anti- phase with each other, and summing the multiplied values, whereby calculation of the correction signal includes a smoothed interpolation. The scaling function of digital to analog converters (10, 12) is used to effect the multiplication and in this case the parameter values are converted to digital form prior to multiplication. <IMAGE>

Description

SPECIFICATION Image correction The present invention relates to image correction and is particularly applicable to image correction in television cameras and monitor.

A television camera has an optical system which directs incident radiation onto a camera tube from which electronic signals are obtained. A number of phenomena occur within the camera for which correction is required if an acceptable quality television picture is to be produced. The number of phenomena requiring correction is considerably increased for colour television cameras.

Such corrections have in the past been applied by manual adjustment of the relevant controls as determined by their effect upon the final image.

Manual adjustment is unreliable, time consuming and generally disadvantageous.

Automatic arrangements have been employed in which a correction signal is applied to the circuitry of the camera or to the electronic signal of the image. These signals are of predetermined form and depend upon the particular phenomena which is being corrected.

Although this technique has been developed so as to introduce complex correction signals, the signals are predetermined in accordance with mainly theoretical considerations.

The corrections applied are calculated for a standard camera tube and therefore any deviation of a tube from the standard will cause a deterioration in performance.

The present invention seeks to mitigate the above disadvantages. According to a first aspect of the present invention, there is provided apparatus for correcting defects in an image having a plurality of picture elements comprising monitoring means for monitoring the value of a parameter from each of the picture elements, processing means for calculating a correction signal from the parameter values and control means for applying the correction signal to correct the image defects.

According to a second aspect of the present invention, there is provided a method of correcting defects in an image having a plurality of picture elements, comprising measuring the value of a parameter at each of the plurality of picture elements, calculating a correction signal from the parameter vlaues and applying the correction signal to correct image defects.

Preferably, calculation of the correction signal includes simultaneous processing of line and field parameter values with the calculation of the signal from the line or field valves including multiplying each of a pair of adjacent parameter values by a respective reference signal, the reference signals being in anti-phase with each other and being equal in frequency to half the frequency of occurrence of the picture elements whose values are being multiplied and summing the multiplied values, whereby calculation of the correction signal includes a smoothed interpolation.

The present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of the interpolation of parameter values, and Figure 2 is a block diagram of a circuit for implementing the interpolation illustrated by Figure 1.

In a television camera there are many features of the image produced which deviate from the ideal arrangement. Often the deviation results in a picture image of an unaccetably poor quality.

Automatic correction of such deviation may be provided by this invention.

The particular feature of the image to be corrected is selected. The correction apparatus is set so as to measure the relevant correction parameter value. The correction parameter value is then measured from each of a plurality of picture elements. The precise method of measuring the correction parameter value depends upon the correction being made, but will usually include comparison of a signal from each picture element with a respective reference signal.

Many features of the television camera will require a correction relating to precise elements of an image as viewed through the optical system of the camera. In such cases a test chart is located in front of the camera. The test chart contains a reguiar matrix of 15 rows and 1 5 columns of rectangular picture areas. The picture areas are identical and are simply areas of black shading on a white background.

The value of the relevant parameter is measured from the signal produced by each of the picture areas on the test card. The measured values are converted to digital form in order to facilitate subsequent processing.

A circuit for monitoring valves from the picture elements may include gate location of the individual picture elements, bandpass filtering, analogue to digital conversion and storage in a RAM under the control of a CPU. The RAM stored valves can be interpolated in real time to provide a correction signal which is applied for real time correction of the image defects.

A correction signal is calculated from the correction parameter values. In calculating the required correction signal it is necessary to process line and field data simultaneously.

Typically, the line rate is of the order of 1 O's of KHz and the field rate seldom exceeds a few hundred hertz. Low pass filtering is possible for line data but is not simultaneously possible for field data since such filtering would result in the loss of information from the line data. It is therefore necessary to use an interpolation technique.

Conventional interpolation techniques used in data processing utilise complex software. Such software can be expensive, difficult to maintain and will sometimes produce discontinuities in interpolation which would result in an unacceptable positional change in the image.

Interpolation of the correction parameter values in this embodiment of the present invention utilizes the scaling function of digital to analog converters.

Each correction parameter value, in digital form, from a pair of adjacent field data picture elements is multiplied by a respective reference signal. The reference signals have a frequency equal to half the frequency of occurrence of field picture elements and the reference signals are in anti-phase with each other. The multiplied values are summed and thereby provide a smoothed interpolation.

Figure 1 illustrates the field rate interpolation of information.

Two reference signals are provided, V REF. 1 and V REF. 2. The frequency of the reference signals is half the frequency of occurrance of the picture elements whose valves are being multiplied. V REF. 1 and V REF. 2 vary linearly between maxima and minima and are in antiphase with each other.

Interpolation between two adjacent field correction parameter values, value 1 and value 2, will now be considered. The reference signals vary between a minimum of 0 and a maximum of V.

The first correction parameter value, value 1, is multiplied by V REF. 1 and the second correction parameter value, value 2, is multiplied by V REF.

2. Initially, V REF. 1 has a value of V and V REF. 2 has a value of 0. At time A in Figure 1 the sum of the correction parameter values multiplied by the respective reference signals is therefore Vxvalue 1. Interpolating between value 1 and value 2, V REF. 1 decreases to 0 while V REF. 2 increases to V. Consequently, when the interpolation is midway between the two values the situation will be as depicted by time B in Figure 1. Namely, the output signal will be equal to V/2x(value 1 +value 2). Having completely traversed from the first picture element to the second picture element, the signals will be as depicted at time C in Figure 1. The output signal is now equal to Vxvalue 2.

The process is then repeated between value 2 and the next value, valve 3. The temporal position of value 3 is represented by D in Figure 1.

A circuit for implementing the interpolation illustrated by Figure 1 is shown in Figure 2. The interpolation utilises the scaling function of digital to analog converters. Two digital to analog converters 10 and 12 are provided. V REF. 1 is applied to an input of the converter 1 0 and V REF.

2 is applied to an input the converter 12. Initially, value 1 is applied to an input of converter 10 and value 2 is applied to an input of converter 12.

Outputs from the converters 10 and 12 are applied to a summing circuit 14. The value applied to each digital to analog converter 10 and 1 2 is changed when the respective reference signal, V REF. 1 or V REF. 2 reaches 0.

Consequently, referring to Figure 1, at time C the value applied to converter 10 is changed from value 1 to value 3. Similarly, at time D the value applied to converter 12 is changed from value 2 to value 4, the fourth correction parameter value.

Variations of the above embodiment will be evident to those skilled in the art. For example, in some circumstances it may be advantageous for the reference signals V REF. 1 and V REF. 2 to have a non-linear form.

Claims (14)

Claims

1. Apparatus for correcting defects in an image having a plurality of picture elements, comprising monitoring means for measuring the value of a parameter from each of the picture elements, processing means for calculating a correction signal from the parameter values and control means for applying the correction signal to correct the image defects.

2. Apparatus as claimed in claim 1, wherein the processing means comprises two multipliers each of which multiplies one of two adjacent line or field parameter values by a respective reference signal which has a frequency equal to half the frequency of occurrance the picture elements whose values are being multiplied, the reference signals being in anti-phase with each other, and a summing circuit for summing the multiplied values, whereby calculation of the correction signal includes a smoothed interpolation.

3. Apparatus as claimed in claim 2, wherein the parameter values are in digital form and each multiplier comprises a digital to analog converter.

4. Apparatus as claimed in claim 2 or 3, wherein the reference signals are linear between maxima and minima.

5. Apparatus as claimed in any preceding claims, further comprising a test chart having a regular matrix of identical picture areas, the picture areas providing respective image elements for the picture elements.

6. Apparatus as claimed in claim 5, wherein the matrix is formed of fifteen rows and fifteen columns.

7. A television camera incorporating the apparatus of any preceding claim.

8. A method of correcting defects in an image having a plurality of picture elements, comprising measuring the value of a parameter at each of the plurality of picture elements, calculating a correction signal from the parameter values and applying the correction signal to correct image defects.

9. A method as claimed in Claim 8, wherein calculation of the correction signal includes multiplying each of two adjacent lines or field parameter values by a respective reference signal which has a frequency equal to half the frequency of occurrence of the picture elements whose valves are being multiplied, the reference signals being in anti-phase with each other, and summing the multiplied values, whereby the calculation includes a smoothed interpolation.

10. A method as claimed in claim 9, comprising multiplying the parameter values, in digital form, and reference signals together in respective digital to analog converters.

1 A method as claimed in Claim 9 or 10, comprising selecting the reference signals to be linear between maxima and minima.

12. A method as claimed in any of claims 9 to 11 comprising the provision of a test chart containing a regular matrix of identical picture areas and arranging the test chart such that the picture areas present an image element for respective picture elements.

1 3. Apparatus for correcting defects in an image substantially as hereinbefore described with reference to the accompanying drawings.

14. A method of correcting defects in an image substantially as hereinbefore described with reference to the accompanying drawings.